In many anti-skid control systems for vehicles, the power pressure used to operate an anti-skid control actuator varies in response to either the brake pressure developed by the master cylinder or the pressure at a master cylinder booster. When master cylinder or booster pressure is low, such as at initial brake application, the actuator operating pressure is also low, so that operation of the anti-skid control actuator in this low pressure range is unstable.
Furthermore, in such systems employing a fail-safe valve operable to develop brake pressure at the wheel brakes when power pressure fails, such as when the power pump fails or a power pressure line ruptures, operation of the fail-safe valve often is unstable and impedes proper operation of the anti-skid control system.